Circuit arrangements including a color display cathode-ray tube of the index type

ABSTRACT

A circuit for ensuring the minimum level flow of current in an indexing type color cathode ray display tube in order to prevent loss of the indexing signal during a scanning period. A gate is provided to prevent application of video signals to the display tube during the first few lines of each field, and during this time a control signal is produced from the indexing signal corresponding to the desired black level. The control signal thus produced is applied to the display tube for the remainder of the field.

358*"69o OR 395629 109 SR D United States Patent 1 1 3 562 409 [721Inventors Kenneth George Freeman 1 56] References Cited 9 y= UNITEDSTATES PATENTS ggggf 2,967,210 1/1961 K6" 1. 178/5 4F 3,201,510 8/1965Davidse v.1,1.,........ .1 178/5.4F gm No 5325 3,234,324 2/1966Mutschler 1, 1. 178/54 [45] Patented Feb. 9 1971 3305,628 2/1967Kitamura 178/5.4F [73] Assignee U.S. Philips Corporation PrimaryExaminer-Richard Murray New York, NY. Assistant Examiner-Richard P1Lange a corporation of Delaware. by mesne Attorney-Frank R. Trifariassignments to [32] Priority June 16, 1967 [33] Great Britain 1]27,918/67 541 CIRCUIT ARRANGEMENTS INCLUDING A 23??? 2 53: f enslirlfg 11 1 i 1 9f COLOR DISPLAY CATHODE-RAY TUBE OF THE c X g ype 9 0 e my u 9"INDEX TYPE order to prevent loss of the 1ndex1ng slgnal durlng a scannmg9 Cl 7D period. A gate is provided to prevent application of video almsrawmg signals to the display tube during the first few lines of each[52] U.S. C1 l78/5.4 field, and during this time a control signal isproduced from [51] Int. Cl H04n 9/22 the indexing signal correspondingto the desired black level. [50] Field of Search 178/52, The controlsignal thus produced is applied to the display tube 5.4, 5.4F, 5.4H forthe remainder of the field.

VIDEO AMPU F 1? H Z z 2 0m PHOTOMVLTIPUER k SNAL5 AMPLIFIER CONTROLAMPIIHEH GATF? J 5 T 1 2 1.0

0x. J ll [GATE REFERENCE FREQUENCy GENERATOR souflcr DIVIDER 5 g t FIELDFLYBACK Puma} PATENTEU FEB sum SHEET 3 0F 4 INVENTORJ KENNETH G. FREEMANMICHAEL C. FRENCH CIRCUIT ARRANGEMENTS INCLUDING A coLo DISPLAYcArnonE-RAY TUBE o r E INDEX TYPE This invention relates to circuitarrangements including a color display cathode ray tube of the indextype. In particular it relates to such arrangements for automaticallycontrolling the amplitude of a signal which would be derived fromscanning the index strips of said tube in the'absence of a video signalapplied thereto. 1

A color display cathode ray tube of the index type normally comprises aseries of vertical strips of phosphor on its screen. These phosphors arearranged to luminesce with different colors when excited by an electronbeam and, in order that the correct phosphor should be excited atalltimes, it is necessary to have information as to the particular phosphorcolor which the beam is exciting at all times. In orderto do this socalled indexing strips are provided between groups of color strips. Thusa signal derived from scanning the indexing strips will provideinformation as to the beam position relative to the color strips. Theindexing strips are usually arranged to emit ultraviolet radiation onbeing scannedlby the electron beam, and this radiation is picked up byan ultraviolet radiation-sensitive element provided inside or outside ofthe tube.

In color television receivers employing such a tube the output of theelement is processed to provide an RF carrier signal which is modulatedwith the color information obtained from the transmitted colortelevision signal. After further processing the signal is applied tothecontrol grid (or the cathode) of the cathode ray tube to provide thecolor drive.

Some indexing tubes employ a pattem 'of indexing strips whose repetitionperiod is notthe same as that of the color strips. For example, thecolor strips may be arranged in groups of three while the indexingstrips may be provided after every two or four color strips. The resultof scanning the screen of such a tube is the provision of an indexingsignal whose frequency is only related to that of the groups of colorstrips and is not equal thereto. A receiver employing such a tube has toinclude inter alia a frequency divider for processing the index signaland this results in a possible phase ambiguity between the output signalof the frequency divider and the indexing signal obtained from theultraviolet-sensitive element. This ambiguity can be overcome bymodifying the structure of the indexing strips on the left-hand side 'ofthe cathode ray tube at a position corresponding in vtime with the startof each line scan of the tube. This modified index structure of socalled frun in strips provides two signals, one at the normal indexfrequency, and another at a lower frequency. Thesetwo signals. enablethe frequency divider to be started, at the beginning of each line scan,always in the same phase relative to the index signal. I

Once the frequency divider has been started at the beginning of the lineof scan it must not be allowed to stop subsequently during that line,since it does so and then restarts its output signal during theremainder of the line may be in an incorrect phase relative to the indexsignal obtained from the element and this will result in incorrectcolors being displayed. j 5

In order to prevent the frequency divider stopping during the course ofthe line the cathode raytube beam currentmust be prevented from fallingbelow the minimum value at which the element provides sufficientoutput'to enable the frequency divider to operate. To obtain a good"black" in the picture this minimum or black level" current shouldnormally be about 2k/ua. Thus the video drive to thetube must be clippedto prevent it driving the cathode ray tube beam current below thisvalue. The black level" current may also be caused to fall below thisvalue by variations in cathode ray tube heater or E.H.T. supply orgrid-cathode voltage variations. As far as the signal seen by thefrequency divideris'concerned the same effeet will occur if, forexample, there is a decrease in the gain of an amplifier supplying itfrom the output of the ultraviolet sensitive element.

It is an object of the invention to provide a circuit arrangement bywhich the minimum value of the input applied to the frequency dividerfrom the element may be maintained substantially constant.

The invention provides a circuit arrangement for automati cally reducingfluctuations which would otherwise occur in the amplitude of a signalwhich would be derived from scanning the indexing strips of a colordisplay cathode ray tube of the index type in the absence ofa videosignal applied to the tube, said arrangement comprising said tube. avideo channel including a gate in series therewith and co pled to acontrol electrode of said tube, means for deriving a control signal fromsaid tube during times when the gate is blocked, which control signalvaries with said amplitude. means for coupling said control signal tosaid tube during each line scan to reduce fluctuations in saidamplitude, and means for blocking said gate during field scansubstantially starting times and rendering it conducting during the scanof the remainder of the lines of the fields on the tube screen.

Thus the circuit arrangement is made to cut off the video signal for aperiod at the start of several, for example each, field scan. This maybe arranged to occur, for example for the first five lines of the scanof each field, or for lines three to ten of these scans, and during thistime the tube current or the indexing signal derived from the tube canbe sampled to obtain the control signal which is then used to maintainthe effective black level constant.

The control signal can conveniently be derived from the output of theindexing amplifier (which normally would feed a frequency divider). Thispoint is the closest one to the frequency divider input which cannormally be used. In this case the field scan starting times mustinclude times when at least a portion of a line lying on the tube screenat substantlally the start of a field is scanned (ovcrscanning beingnormally employed).

The means for deriving the control signal is conveniently only operativewhen the video channel gate is blocked and to this end it may include asecond gate in series therewith. which second gate is arranged toconduct during said field scan staning times and to be blocked duringsaid remainder of the lines of the fields.

An embodiment of the invention will now be described, by way of example,with reference to the accompanying diagram matic drawings in which:

FIG. 1 shows a block diagram of the embodiment;

FIG. 2 shows some alternative points from which the control signal maybe obtained;

FIG. 3 shows a block diagram of the gating signal generato of FIG. 2;

FIG.'4 shows a circuit diagram of the gating signal generator of FIG. 2;

FIG. 5 shows possible input and output waveforms of the gating signalgenerator of FIG. 2;

FIG. 6 shows a block diagram of FIG. 2; and

FIG. 7 shows a circuit diagram of the control amplifier of FIG. 2.

In FIG. 1 a color display cathode ray tube 1 of the index type has avideo channel comprising a video amplifier 2 and a gate 3 coupled to acontrol grid thereof. The tube 1 is provided with ultraviolet-emittingindexing strips (not shown) and a photomultiplier 4 is positioned topick up radiation from these indexing strips. The indexing signalsupplied by the photomultiplier 4 is amplified in an amplifier 5 andcoupled thence inter alia to a frequency divider 6 which supplied anoutput suitable for processing the video signal supplied to the tube inthe normal way.

The index amplifier 5 also supplies a second gate 7 and a gating signalgenerator 8. The generator 8 supplies gating signals in a manner whichwill be described hereinafter to gate 3 and 7 so that the gate 3 isblocked during field flyback times and additionally during the first fewlines of each subsequent field scan on the tube screen (field scanstarting times) and the gate 7 is rendered conducting during these fieldscan starting the control amplifier of times. Thus, the gate 7 feeds theindex amplifier output to a control amplifier 9 during the field scanstarting times (when the video channel gate 3 is blocked). The controlamplifier 9 is arranged to derive a control signal from this indexamplifier output and to apply it, via the video amplifier 2. to thecontrol grid of the tube I to maintain the output of the amplifiersubstantially constant during the field scan starting times. Theamplifier 9 includes an integration circuit so that the control signalsupplied therefrom to the tube 1 is maintained during the scanning ofthe subsequent lines of each field on the screen. The amplifier 9 issupplied with a DC reference voltage from a source in order that themagnitude of the amplifier 5 output may be absolutely determined. Thegenerator 8 is also supplied with the trailing edges of field fiybackpulses by means of a lead 11.

In operation the video drive to the tube 1 is blocked by the gate 3during each field flyback time and for the first few lines of thesubsequent field scan on the tube screen. As soon as the subsequentfield scan on the screen starts the gate 7 is rendered conducting andthus the control amplifier measures the magnitude of the indexing signalsupplied from the photomultiplier 4 and the amplifier 5 and compares itwith the DC reference from the source 10. The signal resulting from thiscomparison is integrated and applied as a DC control signal bias to thegrid of the tube 1 in order to maintain the index amplifier 5 outputsubstantially constant during these first few lines of the field scan.The resulting correction is maintained during the remainder of the fieldscan on the screen by means of the integrating action of the amplifier 9and thus maintains the black level of the video signal subsequentlysupplied to the tube substantially constant and ensures that the outputof the index amplifier 5 to the divider 6 never falls below apredetermined minimum value during each line scan.

FIG. 2 shows some additional circuitry normally provided around the tube1 and alternative positions from which the input to the gate 7 may bederived. As a first alternative the point 12 at the immediate output ofthe photomultiplier 4 may be used. but in this case the resultingcorrection to the "black level" will not take into account anyvariations with time in the gain of the index amplifier 5. The sameobjection would apply if a point 13 were chosen at the output of arun-in amplifier 14. or if a point 15 were chosen from which a sample ofthe cathode ray tube cathode current could be obtained. A point 16 atthe output of the frequency divider 6 could be used but the signalobtained at this point would not vary linearly with the output of theindex amplifier 5 and thus additional complications would be caused.Thus the point 17 at the output of the amplifier 5 is preferred.

FIGS. 3 and 4 show a possible embodiment of the gating signal generator8 of FIG. I. The index amplifier 5 feeds a reset terminal of a bistablecircuit 19 via amplifier 18 which may be tuned. A trigger terminal ofthe circuit 19 is supplied with the trailing edges of field flybackpulses from the lead 11. An output 20 of the circuit I9 feeds the gate 3and an output 21 is coupled to a trigger input terminal of a monostablecircuit 22 having an "on" period equal to the time it is desired to openthe gate 7 each time. The output of the circuit 22 is coupled to thegates 3 and 7 in order to block the gate 3 and to open the gate 7. Thegate 3 is also blocked by the output 20.

FIG. 4 shows a possible circuit diagram of some of the blocks of FIG. 3.The semiconductors can be obtained under the type numbers given underthe Registered Trade Mark Mullard". The signal from the index amplifier5 is fed to the arrangement of FIG. 4 by means of the lead 25. Thecircuit of FIG. 4 includes an extra monostable circuit 23 which may beused if the field time base does not supply a flyback pulse at thecorrect time for triggering the bistable circuit 19. If this is so theflyback pulse from the field oscillator may be fed to the monostablecircuit 23 by means of the lead 24. the trigger pulse for the bistablecircuit appearing at the lead 26. The output of the monostable circuit22 is fed to the gates 3 and 7 by means of the lead 27.

Some possible waveforms appearing in the circuit arrangement of FIG. 4in operation are shown in FIG. 5. together with the index signal(denoted by reference numeral 28) appearing in the index channel. Pulsesappearing on the lead 24 front the field time base are denoted by 29 andthe resulting output from the monostable circuit 23 is denoted by 30.The trailing edge of the output of the monostable 23 results in a pulse31 appearing at the output 20 of the bistable circuit I9. This pulse isbrought to an end by the appearance of indexing signals 32 at the lead25. The trailing edge 33 thus generated gives rise to an output 34 onthe lead 27 from the monostable circuit 22. This output is terminated at35 after a time dependent upon the on" time of this monostable circuit.Thus. the result is that the gate 3 is blocked at the end of the fieldfiyback time by the pulse 31. 33 and then. during the time taken to scanon the tube screen the first few lines of the next field. by the pulse34. 35. During this latter time the pulse 34. 35 also opens the gate 7.The pulses 34. 35 may also be used to remove the elevated run-in signalnormally provided for in indexing circuits. The signal 30 may also beused for field fiyback suppression.

FIGS. 6 and 7 show in more detail how the control amplifier 9 and thesource 10 may be constructed. The output of the index amplifier 5 is fedvia a lead 36 to a further indexing amplifier 37 which incorporates thegate 7 and is supplied with the gating signal from lead 27 (FIGS. 3 and4). The amplifier 37 in turn feeds a first peak detector 38 whichincorporates a comparator for comparing its output with a variable DCreference voltage from a source 39. The detector 38 has an integrationtime long with respect to the line scan period. The comparator output isfed to a second peak detector 40 having an integration time long withrespect to a field scan period. Thus the second detector 40 enables asubstantially DC control voltage to be produced proportional to the peakamplitude of the signal supplied to the detector 40. and hence whichvaries with the signal supplied on the lead 36. This DC signal isamplified in a DC amplifier 41 and in the video amplifier 2 whose outputis DC coupled to the control grid of the tube 1 by means of the lead 42.

In the absence of an index signal. e.g.. when the circuit is firstswitched on. the control amplifier would attempt to establish a veryhigh black level. To limit this uncontrollable black level a diodeclipping circuit 43 is connected to the output of the first peakdetector. The diode is returned to a variable DC reference source (inthis case a further source included in the box 39) so that excursions ofthe control signal supplied by the circuit of FIG. 6 in the directionrepresenting a high black level are limited by the bias voltage appliedto the diode.

The further index amplifier 37 may comprise three synchronously tunedstages with an emitter follower interposed between the second and thirdstages to reduce collector-base feedback effects in the last stage (FIG.7). This circuit may be arranged to give a suitable overall bandwidthsufficient to ensure that its gain is substantially independent ofvariations in the index frequency. The gain of the amplifier isstabilized by employing emitter degeneration and driving each stage froma voltage source. The gating of the amplifier is accomplished byswitching a transistor 44 in the emitter circuit to the final stage.

The output of the amplifier is transformer coupled into the first peakdetector 38 so that the earthy side of the detector can be returned to aset-black-level" control 45. The maximum positive excursion at theoutput of the detector is limited by a diode 46 connected between theoutput and a "set-uncontrolled-black-level" control 47. The integratingcircuit formed by the I00 k0 resistor and 390 pf. capacitor enablesefficient detection of the line component of the input signal to thedetector to be achieved without excessive damping of the output stage ofthe tuned amplifier. The second peak detector may consist of two emitterfollowers shown connected in cascade with an integrating circuitcomposed of a k9 resistor and 200 uuf. capacitor in the emitter circuitof the second stage. The coupling from the detector 40 to the amplifier41 includes an RF filter formed by the 2 K 29m sistor and 4K 7 pf.capacitor in order to ensure that all RF signals may be suppressed atthe input to the amplifier M.

With the circuit of HO. 7 the overall gain may be about I00. The gain ofthe video amplifier 2 (not shown in the diagram of FIG. 7) may be aboutl0 so that the overall gain in the control loop may be about 1,000. Thishas been found sufficient in one receiver to control the index signalamplitude at black level to within i 25 percent of its nominal levelunder all normal operating conditions.

Although the arrangement has been described in isolation it will beevident that it will normally be incorporated in a color televisionreceiver of the index type.

We claim:

1. A circuit arrangement for automatically reducing fluctuations whichwould otherwise occur in the ampiitude of a signal which would bederived from scanning the indexing strips of a color display cathode raytube of the index type in the absence of a video signal applied to thetube, said arrangement comprising said tube, a video channel coupled toa control electrode of said tube, characterized in that the circuitarrangement comprises a gate in series with the video channel and meansfor deriving a control signal from said tube during at least a part ofthe times when the gate is blocked, which control signal varies withsaid amplitude, means for coupling said control signal to said tubedurir ig each line scan to reduce fluc tuations in said amplitude, andmeans for blocking said gate during field scan starting times andrendering it conducting during the scan of the remainder of the lines ofthe fields on the tube screen.

2. An arrangement as claimed in claim 1 wherein said field scan startingtimes are the periods at the start of each field scan when the first fewlines are scanned.

3. An arrangement as claimed in claim 1 wherein the field scan startingtimes include times when at least a portion of a line lying on the tubescreen at substantially the start of a field is scanned, the arrangementincluding means for deriving an indexing signal in response tothescanning of the indexing strips by the tube electron beam, anamplifier for amplifying said indexing signal and a frequency divider,the inputs of said means for deriving a control signal and of saidfrequency divider being coupled with the output of said amplifier.

4. An arrangement as claimed in claim 1 wherein the con- Llt trol signalis arranged to be coupled to the tube via a part of the video channelsubsequent to the video channel gate.

5. An arrangement as claimed in claim 1 wherein the means for derivingthe control signal includes a second gate in series therewith, whichsecond gate is arranged to conduct during said field scan starting timesand to be blocked during said remainder of the lines of the fields.

6. An arrangement as claimed in claim 5 including means for deriving anoutput in response to the scanning of the index' ing strips by the tubeelectron beam and with its output coupled to a reset terminal of abistable circuit, means for coupling the trailing edge of the fieldflyback pulses to a trigger terminal of said bistable circuit, an outputof said bistable circuit being arranged to block the video channel gate,and an output of the bistable circuit being coupled to a trigger inputterminal of a monostable circuit having an on period equal to the timeit is desired to open the second gate each time, the output of themonostable circuit being arranged to block the video channel gate and toopen the second gate.

7. An arrangement as claimed in claim 3 wherein the means for deriving acontrol signal comprises a peak-detector, a DC reference voltage source,and a comparator for comparing the peak-detector output with the DCreference, the resultant circuit coupling the peak-detector output withthe tube having an integration time long compared with a field scanperiod.

8. An arrangement as claimed in claim 7 wherein the peakdetcctor has anintegration time long compared with a line scan period, the comparatoroutput being coupled to said tube via a second peak detector having anintegration time long compared with a field scan period.

9. n a color television receiver of the type having an indexing typecolor display tube, a source of video signals, means applying saidsignals to said tube, means for deriving indexing signals from saidtube, and a source of field flyback pulses; the improvement comprisingmeans connected to said source of flyback pulses and said means forproducing said indexing signal for producing a control signal responsiveto the level of said indexing signals during only predetermined lines ofeach field, said means for applying said video signals to said tubecomprising gate means, means for blocking said gate means during saidpredetermined lines, and means for maintaining current in said tube at apredetermined minimum level comprising means fonapplying said controlsignal to said tube during the remainder of the lines of each field.

1. A circuit arrangement for automatically reducing fluctuations whichwould otherwise occur in the amplitude of a signal which would bederived from scanning the indexing strips of a color display cathode raytube of the index type in the absence of a video signal applied to thetube, said arrangement comprising said tube, a video channel coupled toa control electrode of said tube, characterized in that the circuitarrangement comprises a gate in series with the video channel and meansfor deriving a control signal from said tube during at least a part ofthe times when the gate is blocked, which control signal varies withsaid amplitude, means for coupling said control signal to said tubeduring each line scan to reduce fluctuations in said amplitude, andmeans for blocking said gate during field scan starting times andrendering it conducting during the scan of the remainder of the lines ofthe fields on the tube Screen.
 2. An arrangement as claimed in claim 1wherein said field scan starting times are the periods at the start ofeach field scan when the first few lines are scanned.
 3. An arrangementas claimed in claim 1 wherein the field scan starting times includetimes when at least a portion of a line lying on the tube screen atsubstantially the start of a field is scanned, the arrangement includingmeans for deriving an indexing signal in response to the scanning of theindexing strips by the tube electron beam, an amplifier for amplifyingsaid indexing signal and a frequency divider, the inputs of said meansfor deriving a control signal and of said frequency divider beingcoupled with the output of said amplifier.
 4. An arrangement as claimedin claim 1 wherein the control signal is arranged to be coupled to thetube via a part of the video channel subsequent to the video channelgate.
 5. An arrangement as claimed in claim 1 wherein the means forderiving the control signal includes a second gate in series therewith,which second gate is arranged to conduct during said field scan startingtimes and to be blocked during said remainder of the lines of thefields.
 6. An arrangement as claimed in claim 5 including means forderiving an output in response to the scanning of the indexing strips bythe tube electron beam and with its output coupled to a reset terminalof a bistable circuit, means for coupling the trailing edge of the fieldflyback pulses to a trigger terminal of said bistable circuit, an outputof said bistable circuit being arranged to block the video channel gate,and an output of the bistable circuit being coupled to a trigger inputterminal of a monostable circuit having an ''''on'''' period equal tothe time it is desired to open the second gate each time, the output ofthe monostable circuit being arranged to block the video channel gateand to open the second gate.
 7. An arrangement as claimed in claim 3wherein the means for deriving a control signal comprises apeak-detector, a DC reference voltage source, and a comparator forcomparing the peak-detector output with the DC reference, the resultantcircuit coupling the peak-detector output with the tube having anintegration time long compared with a field scan period.
 8. Anarrangement as claimed in claim 7 wherein the peak-detector has anintegration time long compared with a line scan period, the comparatoroutput being coupled to said tube via a second peak detector having anintegration time long compared with a field scan period.
 9. In a colortelevision receiver of the type having an indexing type color displaytube, a source of video signals, means applying said signals to saidtube, means for deriving indexing signals from said tube, and a sourceof field flyback pulses; the improvement comprising means connected tosaid source of flyback pulses and said means for producing said indexingsignal for producing a control signal responsive to the level of saidindexing signals during only predetermined lines of each field, saidmeans for applying said video signals to said tube comprising gatemeans, means for blocking said gate means during said predeterminedlines, and means for maintaining current in said tube at a predeterminedminimum level comprising means for applying said control signal to saidtube during the remainder of the lines of each field.